Existing transfer switches may be used to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. The transfer switches utilize either contactors or circuit breakers that are able to withstand a high current for switching loads. The transfer switch contactor has a set of contact arrangements to switch from one source to another. These contactors are usually operated by means of an operating mechanism that is mechanically linked to the contactor assembly.
Under short circuit conditions very high amperages are developed. Currents as high as 5,000 amperes may develop under some short circuit conditions, while under more severe and extraordinary conditions, currents as high as 20,000 amperes or in excess of that may occur.
In prior devices a pair of fixed contacts were bridged by a movable contact that was held in closed circuit condition by a spring pressure against the movable or bridging contact. Ordinarily, the fixed contact members and the bridging contact members as well as the contact surfaces themselves are massive and are held closed by relatively heavy springs which impose considerable pressure on the bridging contact to hold it in engagement with the fixed contacts.
On the occurrence of the short circuit current over a threshold amperage, contact pressure of the spring holding the bridging contact in engagement with the movable contact can be overcome by magnetic forces of repulsion developed between the stationary and movable contacts. When the spring bias is overcome, the contacts tend to separate. Contact separation under short circuit conditions results in arcing. This sometimes has resulted in a violent explosion in instances of a large fault. In less severe cases there may be a melting of small amounts of contact material, with the result that when the “short” was cleared, by a fuse, circuit breaker, or other protective device, the contacts would snap back together and the molten material would cool. This caused firm welding of the contacts together.